In addition to endoscopes with medical and non-medical technical applications whose viewing angle is parallel to the longitudinal axis of the endoscope shaft, endoscopes with other fixed viewing angles were developed for some time in the past. The viewing angle of an endoscope is understood here and hereinafter always to refer to the angle looking from the distal end of the endoscope, in which an object lies that appears in the center of the image recorded in each case by the endoscope. In many applications, however, a fixed viewing angle is a disadvantage. In the most unfavorable case, for example, the endoscope must be replaced repeatedly during a medical procedure. In such cases the use of an endoscope with an adjustable or displaceable viewing angle in situ is advantageous.
Observation of an object in a cavity by means of an endoscope, as a rule, requires the object to be illuminated. For this purpose an endoscope comprises, for example, lightwave conductors, in particular glass fibers, by means of which illuminating light is transmitted from the proximal end of the endoscope along the shaft to the distal end of the endoscope. Light outlet surfaces of the lightwave conductors on the distal end of the endoscope are positioned and configured in such a way that the entire visual field or viewing field is sufficiently illuminated.
In an endoscope with adjustable viewing angle, the illuminating light on the distal end of the endoscope is distributed, in the simplest case, in such a way that, depending on the particular viewing angle selected, the entire visual field is illuminated. This leads to a series of disadvantages, however. In particular, light capacity is wasted because the entire viewing fields of all adjustable viewing angles are constantly illuminated, independently of the viewing angle actually selected. At a predetermined desired brightness, a markedly higher light capacity must thus be made available than with an endoscope with fixed viewing angle.
An additional disadvantage is that illuminating light of higher intensity can photothermally or photochemically harm tissue or other objects. With an endoscope of fixed viewing angle, as a rule too little distance occurs from the distal end of the endoscope to an object, at least in observing the recorded image. In using a video camera on the endoscope, an automatic warning of users is also possible when the brightness of a recorded image exceeds a predetermined threshold. In an endoscope with adjustable viewing angle, however, part of the illuminating light impinges on objects situated outside the visual field. Therefore undesired approach of the distal end of the endoscope to these objects and a resulting radiation of these objects with too high a radiant capacity are avoided.
A further disadvantage consists in the fact that illuminating light that is at first radiated outside the visual field can be scattered or reflected by objects or opaque media. Reflected or scattered illuminating light can reach the observation beam path directly or indirectly. Consequently, contrasts and especially in dark image areas the distinguishability of objects can be reduced. In addition, visible, disturbing reflections can thereby be generated.
An additional disadvantage comes from the fact that the illumination strength, or intensity, of the illuminating light is essentially constant in the direction in which the viewing angle can be varied (often referred to also as the vertical direction), while in the direction perpendicular thereto (often called the horizontal direction) as a rule it slightly declines toward the edge of the visual field. From endoscopes with fixed viewing angle, however, users are as a rule accustomed to an illumination strength that slightly declines toward the edge of the visual field both in the horizontal and in the vertical direction. Therefore the illumination strength that is constant in the vertical direction can be experienced as an irritation.
Patent DE 600 15 375 T2 describes an arrangement of two prisms. One of said prisms can rotate around an axis to cast illuminating light at an adjustable viewing angle. The inventors of the present invention, however, have determined that the distribution of the illuminating light inside the visual field with the described arrangement of prisms is unsatisfactory in many cases. This distribution is determined by the light source or the fiber bundle and cannot be influenced by the prisms.